A known microwave ceramic filter consists of at least one resonator designed in a dielectric ceramic base body. The base body therefore has at least one hole, the inside walls of which are metallized. The outside walls of the ceramic base body are also metallized except for one end face and are contacted with the metallic hole on the short-circuit end, which is opposite the non-metallized end face. Electric terminal faces which are used for direct or indirect capacitive coupling to the metallized hole, forming the actual resonator, are provided on the bottom side or on one end face, where they are electrically isolated from the exterior metallization.
Microwave ceramic filters have the advantage that they can be designed comparatively easily and manufactured inexpensively.
Furthermore, they have a low insertion loss.
U.S. Pat. No. 5,686,873 discloses a monolithic duplexer. A single ceramic body is provided, containing a transmission branch (TX path) and a reception branch (RX path), each composed of a plurality of coupled resonators. The advantage of such monolithic duplexer lies in its manufacture: only one ceramic body need be pressed in one piece, which greatly simplifies fabrication in comparison with the manufacture of two ceramic bodies.
In known microwave ceramic duplexers, the individual signal paths of the RX path and the TX path are unbalanced so that each input and output is designed to be single-ended. However, there are already demands on the part of chip manufacturers and telephone manufacturers to design the RX path for future UMTS systems to be balanced already starting with the duplexer. Such a symmetrical output requires two terminals at which two signals having opposite polarity, i.e., ideally with a 180° phase shift can be picked up. Such balanced outputs are to be sent in particular to simple modern amplifiers for the RX system.
A balanced output signal cannot be supplied by a microwave ceramic duplexer without increasing the size of the component and providing additional holes. This usually requires an external balun, while other filter techniques such as SAW filters and duplexers may contain the balun functionality integrated into the filter design.
Microwave ceramic duplexers have an extremely low insertion loss in contrast with the former, with a larger design in comparison with SAW filters, and they have a good electric and acoustic adjustment. The low insertion loss is especially interesting for UMTS systems with regard to the possible lengthening of the operating time of the battery. The combination of a microwave ceramic (MWC) duplexer with a separate balun on a circuit board results either in a greater base area being required or the allowed overall height of the component being exceeded; at the present time, the overall height is based on MWC duplexers without a balun. However, a corresponding reduction in the height of the duplexer results in a considerable sacrifice in terms of performance and in particular an increase in the insertion loss in both branches.